The present invention relates to testing of motor operated valves, and more particularly to apparatus for testing the spring pack used in such valves to verify that the spring pack is functioning with acceptable specifications.
Many motor operated valves contain a spring pack which is biased whenever torque is applied to the valve stem by the motor. A torque switch is coupled to the spring pack so that when a certain level of torque is reached, as occurs in the fully opened or fully closed positions of the valve, the torque switch stops the motor and prevents the application of more torque. Should the spring pack not conform to specifications, the torque switch will be inappropriately triggered. If the switch is triggered too early, the valve may not be in its fully open or fully closed position, thereby affecting flow through the valve. If the switch is triggered too late, the motor will be allowed to apply additional torque to the valve components, possibly damaging those components. Thus, it is important for the spring packs to accurately operate within. their specified range.
In the past, spring packs typically were removed from the valve operator and placed in a special test stand to evaluate their performance. The test stand compressed the spring pack while measurements of the force exerted and the spring pack movement were recorded. This allowed a graph to be created of the spring pack force versus displacement. An example of a spring pack testing apparatus of this type is shown in U.S. Pat. No. 5,167,151. Afterwards, the data was evaluated either by itself or by comparing it to previous tests conducted on the same spring pack to determine changes with time. Depending upon the results of the evaluation, the spring pack was either re-inserted into the motor operated valve or discarded.
Motor operated valves are often used in very critical systems, such as those which control the flow of coolant in nuclear reactors used in electric utility generating stations. Very stringent requirements are placed on motor operated valves in these critical applications. In such installations, when the spring pack was re-inserted into the valve after testing, the operating mechanism had to be recalibrated and thoroughly inspected to insure proper functioning of the valve. This not only was time consuming procedure, but also expensive.
U.S. Pat. No. 4,860,596 shows an alternative method for testing the motor operated valve in which a load sensor was attached to a shaft which operated the torque limit switch. Although this type of tester eliminated the need to disassemble the valve operator and remove the spring pack, it did not directly measure the force exerted on the spring pack and further required that the valve be cycled through open and closed states. It is desirable in many installations to be able to test the spring pack directly to obtain measurements of its actual performance and without operating the valve so that the state of the fluid flow is unaffected during testing.